Have found an American Classic bought direct, before (close-out).

I have also used an Adjustable reamer to shave metal out of the inside of the seat tube, *
to , in my case , take a 27.0 ID frame out to 27.2 to use a more common seatpost size..

Happen to have found the saddle clip Brompton makes 'the Penta Clip" is the best of any Ive seen.

mostly aluminum.. the angle is held with a multiplate friction washer set, so stepless and solid with 1 bolt..

then a common Kalloy plain top post would do fine.. functionally , aesthetically I cannot say your requirements ..
but functionally they are excellent.

there is also the buy smaller and shim, out from there to 26.4..

I own a Campag Super record Seatpost, left over from owning an AlAn. its 25.0.
I had used a 25~27,0 shim (USE, UK machined) in the above frame ^*
Wheels Mfg & QBP also sell shims to resize seatposts..